Fumes Treatment System

ABSTRACT

A fumes treatment system is provided for exhaust gases of internal combustion engines, for example. The system includes a housing inside which a series of elements are provided for intercepting the flow of fumes. The elements are such that they do not completely take up the internal cross-section of the housing and are provided with or leave such discontinuities as to create a tortuous path for the fumes to be treated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Section 371 of International Application No.PCT/IT2007/000095, filed Feb. 14, 2007, which was published in theEnglish language on Aug. 23, 2007, under International Publication No.WO 2007/094024 A1 and the disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a system for the treatment of fumesderived from combustion; in particular, the system of the invention mayfind application in the treatment of fumes derived from the combustionof hydrocarbons.

The fumes emitted from the combustion of hydrocarbons are among the maincauses of atmospheric pollution, and the regulations on their controland suppression are becoming more and more stringent. The main sourcesof such a kind of emissions are boilers of heating equipment andinternal combustion engines of transport means. Reference will be madein the rest of the text to application to transport means, but theinvention may be also applied in the field of boilers or central heatingsystems as well.

The emissions from the combustion of hydrocarbons are mainly comprisedof (other than aqueous vapor) CO₂, CO, NO₂, NO, sulphur compounds, andunburned hydrocarbons. In addition, gasoline engines cause emissions ofbenzene (employed for many years as an antiknock substance in place oftetraethyl lead), whereas diesel engines emit a solid particulate,generally indicated in the field by the definition of “soot.” Soot ismade up of particles of a size between about 50 and 1000 nanometers (nm)and formed by carbon residues generated by the incomplete combustion ofthe fuel, on the surface of which condensed hydrocarbons together withother substances are generally adsorbed. These particles aresufficiently thin to remain suspended in air for relatively long periodsof time. In the rest of the text by the term “fumes” is meant thecombination of gaseous compounds, vapors and soot.

In order to suppress all these species, vehicles are provided with gasand particulate treatment systems generally comprised of a housing(normally metallic) connected to the engine at one end and to theexhaust at the other end. Inside the housing, elements are arranged thatare active in the conversion of gaseous species into less harmfulspecies, and filters for retaining soot. The arrangement of convertersand filters in the housing is such that gases and particulates have topass through these active elements, or at least lap on them.

Particulate filters are generally periodically regenerated by causingthe accumulated soot to burn, in order to avoid their clogging, whichwould impair their proper operation. This operation is commonly carriedout according to the so-called “post injection” method, wherein theexhaust cycle of the pistons is temporarily modified in order to cause“fresh” fuel to arrive in the exhaust line, which by burning downstreamof the engine increases the exhaust gas temperature up to about 650° C.,thus triggering the combustion of the soot accumulated on the filter.The operations of gas conversion and soot combustion are generallyfacilitated by the presence, on the elements forming these systems, ofoxidation catalysts that have the function of reducing the combustiontemperature of undesired species (or, which is the same, the function ofincreasing the oxidizing efficiency at a given temperature).

The treatment systems of exhaust gases presently in use are mostly basedon the use as active elements of the so-called ceramic monoliths, i.e.,porous ceramic bodies that may be made, for example, of silicon carbideor cordierite. Monoliths generally have a structure known in the fieldas “honeycomb,” formed of a series of parallel channels. In particular,filters are formed of two series of blind channels, wherein the channelsof the first series are open only at the side of the inlet of the gasesto be treated and those of the second series are open only toward theside of gas exhaust. The two series are alternated so that (in across-section of the monolith) a channel of one series has the channelsof the other series as first neighbors. With this construction, thegases are obliged to pass through the walls separating the channels,exploiting the porosities of the material, in order to go from the inletto the outlet of the treatment system. In order to improve the treatmentefficiency, the surfaces of the channels, as well as those of the pores,are generally catalyzed as described above.

However, these monoliths exhibit some problems.

First of all, gases coming from an engine are forced to pass through theporosities of the material and this leads to relatively high values ofpressure drops at the two ends of the monolith (the pressure drop isusually indicated in the field as “head loss”; this definition will beadopted in the rest of this specification). The situation is worse whenthe channels are clogged by soot before a regeneration. A large headloss across the monolith results in a reduction of the power generatedby the engine, and thus in the need for higher fuel consumption in orderto have the same vehicle performance. In extreme cases, when the headloss becomes too high, it may lead to a spontaneous shutoff of theengine.

A second problem of ceramic monoliths is their inherent fragility. Atevery turning on and off of the engine, these monoliths are subject tosudden and intense temperature excursions between ambient temperatureand temperatures of about 400° C., depending on the distance betweenmonolith and engine. During the combustion steps of the soot, evenhigher temperatures are achieved, which, due to the poor thermalconductivity of the ceramics, can create strong thermal gradients in themonolith structure. Moreover, during the motion of vehicles, themonoliths are subject to intense mechanical stresses. The combination ofthese phenomena may lead to the formation of cracks in the monolith,which are preferential paths for the gaseous flow crossing them, thusreducing the efficiency of gas conversion or soot retention.

In addition, since in these systems the fumes must pass through theceramic walls separating the channels, in order to have acceptable headlosses at the ends of the system, monoliths having high values of totalsurface area are manufactured, usually equal to some square meters,depending on the application. This entails various problems: first, thesize of these systems may cause positioning problems in the exhaustsystems of the transport means; second, the use of relatively largeamounts of material may lead to expensive fumes treatment systems,particularly when using silicon carbide for the filters and noblemetals, such as platinum or palladium, as a catalyst in the gasconverters.

In order to overcome these problems, the use of converters or filtershas been suggested, wherein porous elements made of metallic materialare used, instead of ceramic elements. These may be made of preformedthin metal plates, sintered metal powders or metal fibers packed andpossibly adhered one to another by means of thermal treatments.

One type of active metal elements is those made from metal fiber meshes.The preferred material for this purpose is an alloy called Fecralloy,essentially formed of iron (the main component), aluminum, chromium andyttrium, the composition and the preparation of which are disclosed inU.S. Pat. No. 3,920,583. A suitable method for producing large amountsof Fecralloy fibers is disclosed, e.g., in U.S. Pat. No. 4,930,199. Thisalloy is particularly suitable for this application, as, exposing it toan oxidizing atmosphere at high temperatures (around 1000° C.), amigration of aluminum to the surface occurs with formation of a layer ofalumina that is compact and highly resistant to thermal shocks,mechanical stresses and chemical attacks. As a consequence, after thisfirst exposure to oxidizing conditions, the material becomes extremelyresistant to the conditions in which it will be in the systems for thetreatment of exhaust gases, regardless of how aggressive they may be.

On this thermally grown layer of alumina, a second less compact oxidelayer is then preferably deposited, which, in turn, may be made ofalumina, a mixed oxide of aluminum and Rare Earth elements, titaniumoxide, or the like. This second oxide layer may be formed in many ways,for example by dipping the mesh into a solution containing precursors ofthe oxides and successive thermal treatments for evaporating thesolvent, transforming the precursors into oxides and consolidating thelatter. Another suitable process for this purpose is disclosed in U.S.Pat. No. 6,303,538 B1. Finally, this porous oxide layer may be madefunctional by a catalyst, which is generally a noble metal, such asplatinum, palladium, rhodium or mixtures thereof, or oxides such asvanadium pentoxide, lanthanum manganate or cerium-zirconium mixedoxides. The preparation of a Fecralloy mesh catalyzed by acerium-zirconium mixed oxide is disclosed, e.g., in European Patent EP0764455 B1.

These meshes may be arranged in the housing in a direction substantiallyparallel to the axis thereof. In this case, bellows configurations arepossible, as disclosed, e.g., in European patent application publicationEP 0504422 A1, or having a cylindrical symmetry, as disclosed, e.g., inU.S. Pat. No. 4,576,799 or in European patent application publication EP0699827 A1. With these configurations two series of spaces are generatedalternately open at the gas inlet and outlet portions, similarly to whatis present in the ceramic monoliths. The gases are forced to passthrough the meshes in order to go from one space of the first series toone of the second series, but all the spaces of these two series areessentially equivalent to each other from the point of view of the flowcharacteristics, and thus the meshes in these configurations aregenerally all equivalent. Alternatively, the meshes may be arrangedtransversely to the axis of the housing, so that a series of successivespaces is created along the flow direction, as disclosed, e.g., in U.S.Pat. No. 4,900,517. In this case, it may be preferable to use meshes ofdifferent characteristics to manufacture successive elements of theseries, for example having a more and more decreasing porosity, in orderto try to obtain a degree of gas conversion and soot retention ashomogeneous as possible over all the active elements arranged in seriesin the housing, as described, e.g., in U.S. Pat. No. 5,968,373.

In any event, in all the constructive solutions described in the priorart, fiber meshes are connected gas-tight to the inner walls of thehousing (generally by metal supporting elements), so as not to leaveside passages, so that the stream of gases and particulates is forced topass through the meshes along the path from the inlet to the outlet ofthe housing. This arrangement assures a good contact between the fumesand the active elements, but causes also a high head loss.

In addition, this structure has the problem that meshes can accumulatesoot, resulting in the reduction of the free area for the gas passage,clogging and increase in the head loss across the ends of the system.Finally, the inventors have verified that with a treatment system basedon meshes arranged along the filter axis, of the type described inEuropean patent application publication EP 0504422 A1, in order to havea filter with gas and particulate treatment characteristics comparableto ceramic monoliths, about 0.6-0.7 m² of metal mesh are needed. Giventhe high cost of these metal meshes, this leads to costs of thesesystems that are higher than those based on ceramic monoliths.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to overcome the problems of theprior art, and particularly to provide a system for the treatment offumes from internal combustion engines that has a reduced head lossand/or a reduced amount of material and catalyst with respect to knownsystems, with the same treatment efficiency.

According to the present invention, this object is achieved by a fumestreatment system comprising a housing provided with a fumes inlet and anoutlet of the treated fumes, and inside the housing a series ofessentially planar flow interception elements, characterized in that theelements are formed and arranged so that:

at least two successive interception elements of the series are eachprovided with at least one discontinuity, such that the geometrical areaof the discontinuities is between 10 and 40% of the area that theelement would have if it completely occupied the internal cross-sectionof the housing;

for any pair of successive interception elements presentingdiscontinuities, the condition exists that the projection of thediscontinuity of the downstream element on the upstream one, theprojection being made perpendicular to the upstream element, does notsuperimpose with the discontinuity on the upstream element; and

the minimum distance between two discontinuities on two successiveelements lies along a segment forming an angle of at least 5° withrespect to a straight line perpendicular to the upstream element.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown. In thedrawings:

FIG. 1 is a series of perspective views of different possible shapes ofinterception elements for use in the systems of the invention;

FIG. 2 is a schematic representation of geometric condition b)characterizing the invention in its most general embodiment;

FIG. 3 is a schematic representation of geometric condition c)characterizing the invention in its most general embodiment;

FIG. 4 is a schematic cross-sectional view of a preferred embodiment ofthe system of the invention;

FIG. 5 is a perspective view of a schematic illustration of thegeometric conditions characterizing the invention in a preferredembodiment;

FIGS. 6 and 7 are partially broken away views of two possible systems ofthe invention comprising different types of flow intercepting elementsand their mutual arrangement;

FIG. 6 a is an enlarged view of a detail of FIG. 6; and

FIG. 8 is a diagrammatic view of an experimental set-up for testing theparticulate filtration efficiency of filtering systems.

DETAILED DESCRIPTION OF THE INVENTION

The flow interception elements used in the systems of the invention arealmost always porous bodies, and thus inherently discontinuous, but inthe present specification and in the claims by discontinuity is meantdiscontinuities of the macroscopic type, i.e., holes in the surface ofthe elements, or elements having such a geometrical shape that they donot completely take up the cross-section of the housing.

The inventors have found that, contrary to what would seem intuitive andto the common manufacturing principles of prior art systems, it ispossible to manufacture fumes treatment systems, wherein at least somesuccessive flow interception elements are provided with openings oranyway with portions not contacting the housing walls, provided thatthese are arranged so as to impose a tortuous path to the fumes, withoutlosing treatment efficiency (gas conversion and particulate retention),and with reduced head losses with respect to similar systems in which,in contrast, all the elements completely take up the cross-section ofthe housing. In fact, the increase in turbulence imposed by the tortuouspath enhances the contact between the fumes and the active elements,thus compensating the reduction in efficiency caused by the presence ofthe discontinuities.

FIG. 1 shows several possible shapes of discontinuous elementssatisfying condition a) of the invention. Element 10 is formed as acircle missing a portion, element 11 has a central hole 12, whileelement 13 presents a series of holes 14 near its periphery. Theseelements are represented as ready to fit in a housing of essentiallycircular or elliptical cross-section, but of course many other shapesare possible for the cross-section and consequently for the overallshape of the elements.

FIG. 2 represents condition b) of the invention. The drawing shows incross-section a housing 25 of the system of the invention, wherein twodiscontinuous elements are positioned. In the drawing the flow of fumesis assumed to move from right to left, so that the element at the rightis the upstream one. The downstream element is shaped as element 10 ofFIG. 1, while the upstream element is shaped as element 13 withdiscontinuities 14 of FIG. 1. The downstream element defines with thewalls of housing 25 a discontinuity indicated in the drawing by numeral26. The projection (indicated by P in the drawing) of this discontinuityonto upstream element, drawn along lines perpendicular to the latter,has no superimpositions with discontinuities 14.

Finally, FIG. 3 represents condition c) of the invention as realized inthe same system of FIG. 2. Point A in FIG. 3 represents the edge of thedownstream element, while point C on the upstream element is theprojection of point A onto the latter element, drawn along a lineperpendicular to the upstream element. Discontinuity 14 in the upperpart of element 13 in the drawing is the closest one to discontinuity26. The segment joining point B of this closest discontinuity on element13 to point A on the downstream element is the minimum distance betweentwo discontinuities on the two successive elements. The angle α formedby segment A-C (lying on a straight line perpendicular to the upstreamelement) and segment A-B (on which lies the minimum distance betweendiscontinuities as described above) must be at least 5°.

Though the invention may be realized by disposing the interceptionelements in the housing in the most varied arrangements, for instancenon-perpendicular to the axis of the housing and not parallel to eachother, the preferred embodiments of the invention are those in which theflow interception elements are arranged essentially perpendicular to theaxis of the housing and essentially parallel to each other. In thispreferred configuration, a line perpendicular to the upstream element ina pair of successive interception elements with discontinuities isessentially parallel to the axis of the housing, so that:

geometrical condition b) of the invention results in the requirementthat for any given pair of successive discontinuous elements, looked atalong a line parallel to the axis of the housing, no optical path existsallowing a sight through the pair of elements;

while geometrical condition c) of the invention results in therequirement that the minimum distance between two discontinuities on twosuccessive elements lies along a segment forming an angle of at least 5°with respect to a straight line parallel to the axis of the housing.

Moreover, the invention does not necessarily require that all flowinterception elements in the housing of the system be provided withdiscontinuities. For instance, it is possible to resort to hybridsolutions, where some elements of the series are of a traditional type,i.e., full and sealed against the inner walls of the housing, in orderto increase the soot retention or the efficiency of gas treatment.However, a system wherein all of the interception elements showdiscontinuities is preferred in view of head loss reduction.

Finally, the invention does not require that the flow interceptionelements be placed equidistant in the housing or have a comparableoverall size of the discontinuity. For example, it is possible to havesystems where the size of the discontinuities, or the distance betweensuccessive elements, decreases from the inlet to the outlet, in order tohave the fumes treatment efficiency increasing in this order. Thisallows achievement of a more homogeneous treatment efficiency throughoutthe system.

Notwithstanding the above described possible freedom of construction,from a practical standpoint the systems where all flow interceptionelements are essentially perpendicular to the axis of the housing andessentially parallel and equidistant to each other, and each of theelements is provided with discontinuities, represent the preferredembodiments of the invention, in particular in view of ease ofmanufacturing. In the following part of the description, reference willbe made to these preferred conditions and embodiments, unless stated tothe contrary. On the other hand, the systems described in detail in thefollowing do not necessarily comprise flow interception elements havingcomparable dimension of the discontinuities.

FIG. 4 shows in cross-section a possible preferred embodiment of asystem of the invention for the treatment of fumes. System 40 comprisesa housing 41 generally made of metal (e.g., stainless steel) comprisedof a main chamber 42 tapered at the ends in correspondence to twotubular portions 43 and 43′, respectively used to connect the system toa pipe for the fumes coming from the engine and to a pipe for theexhaust of the gases to the outside. In the drawing, the fumes cross thesystem from the right to the left. Chamber 42, in a view along the axisof the housing, commonly has a circular or elliptical cross-section, butother forms are also possible. In chamber 42 flow interception elements44 are present, that do not completely take up the cross-section ofchamber 42. Besides, the elements 44 are arranged in housing 41 so thatthe discontinuities formed by two adjacent elements with the inner wallof chamber 42 are essentially symmetrical with respect to the axis (notshown) of the housing.

By this construction, a net path for the fumes is obtained (representedby the arrows in the drawing), such that the latter are not necessarilyforced to pass through the material of element 44 in order to go fromthe inlet to the outlet of the system. On the other hand, thetortuousness of this path results in a high turbulence of the fumes,causing them to contact the surfaces of elements 44 (both the surfacefacing the inlet and the one facing the outlet) and thereby the catalystmaterials arranged on the surfaces and in the porosities of elements 44in case the system 40 is used as a gas converter, or causing them tocontact the soot-retaining surfaces in case the system is used as aparticulate filter.

The elements 44 may be held in place by fixing elements of many types:for example, metal parts may be employed, having a height equal to thedesired distance between two successive elements 44, these parts beingsections of a hollow cylindrical-shaped body (where the term cylinder isused in the broad meaning of a surface generated by a straight linemoving parallel to itself along a closed curve) having the externalsurface of shape and size essentially equal to the internal surface ofchamber 42. FIG. 4 shows fixing elements 45 of this type, but suchelements may be made with many different shapes, which will be evidentto those skilled in the art.

FIG. 5 schematizes the geometrical condition characterizing theinvention in the preferred embodiment of elements parallel to each otherand perpendicular to the axis of the housing. In the drawing, twosuccessive flow interception elements are shown, arranged in a circularcross-section housing (not shown) and having a generic shape of thediscontinuities. In particular, element 50 has the shape of element 10in FIG. 1, while in element 51 the discontinuity has the shape of acircular aperture 52, which does not reach the edges of the element andthus the walls of the housing. The straight line X-X′ represents theaxis of the housing. The minimum distance that may be identified betweenthe two discontinuities in elements 50 and 51 is represented by segmentD-D′, where D and D′ are two points on the edge of the discontinuitiesof elements 50 and 51 respectively, while the straight line Y-Y′ isparallel to X-X′ and passes through point D′. The angle α formed bysegment D-D′ and straight line Y-Y′ is the angle characterizing thesystems of the invention, which must be equal to at least 5°.

Obviously, the value of this angle is determined by the size of thediscontinuities on adjacent elements and by the distance between theseelements. Therefore, it is possible to adopt configurations withelements having discontinuities of a greater size by moving the elementscloser to one another, or to have elements being farther apart havingdiscontinuities of a smaller size, as long as the condition α≧5° ismaintained. The value of this angle is preferably greater than 15°, andmore preferably greater than 30°.

In contrast, the maximum value of angle α is not fixed, but it isvariable mainly depending on the material forming the flow interceptionelements and, in particular, depending on their porosity. In thepreferred embodiments (elements parallel to each other) this maximumangle must be less than 90° (an angle of 90° would mean that twosuccessive elements are in contact with each other). Preferably, themaximum value of angle α is not greater than 85°, because for greatervalues the turbulence generated in the gas flow is such as topractically suppress the advantage of head loss reduction achievedthrough the discontinuities.

FIG. 6 and the enlarged view of a detail thereof in FIG. 6 a show in apartially broken away view a possible embodiment of the system of theinvention, wherein the flow interception elements are provided withdiscontinuities at the internal wall of the housing. The system 60 iscomprised of a circular cross-section housing 61 provided with an inlet62 for the fumes coming from the engine and of an outlet 62′ for thetreated fumes. Inside the housing flow interception elements 63 arearranged, held in place by ring-shaped fixing elements 64. The positionof the latter with respect to the flow interception elements ishighlighted in the enlarged view of FIG. 6 a, corresponding to theportion of the drawing of FIG. 6 delimited by a dashed circle. Theelements 63 are oriented in such a way that the discontinuities 65 areas far from each other as possible. In practice, this is achieved bymaking the chords defining the segments of the discontinuities parallelto one another and alternated on successive elements with respect to theaxis of the housing.

FIG. 7 shows in a partially broken away view another possible embodimentof the system of the invention. In system 70 the flow interceptionelements are provided with discontinuities in the form of aperturescompletely included within the geometrical area of the elements, so thatthe apertures do not reach the inner wall of the housing 71. In thedrawing, for ease of illustration, fixing elements similar to elements64 of FIG. 6 are not shown. In this case, the flow intercepting elementsare of two types: the first type, corresponding to elements 72, exhibitsa central circular aperture 73; the second type, corresponding toelements 74, exhibits a series of apertures at the periphery of theelement (together referred to by number 75 or 75′). The apertures on theelements of the second type are preferably equidistant and circular forease of manufacturing (apertures of type 75), but they could also haveother shapes, e.g., elongated (apertures of type 75′).

The materials with which the flow interception elements can be made maybe the most varied, particularly depending on the intended use of thetreatment system. If the main purpose is gas conversion, the elementswill generally be rather porous in order to enable gases to penetratethe porosities and come into contact with the catalyst materialdeposited on the surfaces of the elements, including the inner surfaces.In the case where the intended use of the system is mainly that of asoot filter, these elements are generally less porous than those of theprevious case, and they could even be made of solid metal plates, aslong as they are associated with layers of an open structure material,such that it can collect the soot.

In the case where the system is a gas converter (e.g., to be used as acatalytic system immediately downstream of the engine, in theconfiguration known as “Close Coupled Catalyst” or CCC), theinterception elements are preferably made of metallic material, e.g.,Fecralloy alloy or AISI 310S steel, in the form of a fiber mesh, aporous body of sintered powders or a foam. Porous bodies in the form ofmetallic foams, e.g., made of Fecralloy, are sold by the PorvairAdvanced Materials Company of Hendersonville, N.C., USA. The fibers maybe obtained by the process disclosed in U.S. Pat. No. 4,930,199. It isalso possible to employ multi-layer fiber mats in which the layers havedifferent fiber sizes and porosities, as disclosed, for example, inEuropean patent EP 1 450 929 B1, to be preferably employed in such anorientation that the layer facing the fumes inlet has greater size offibers and porosity. Alternatively, it is possible to employ ceramicelements, e.g., made of silicon carbide, cordierite or aluminumtitanate.

In the case where the system of the invention is to be essentiallyemployed as a soot filter (for example downstream of a catalyticconverter and in a colder position), it is possible to use in themanufacture of the interception elements all the materials previouslycited for the converter, preferably with a size of the porosities lessthan that of the materials used for manufacturing the elements of thecatalytic converter. In this case, it is further possible to usenon-porous materials, e.g., a solid metal plate, if the latter areassociated with materials having porosities or apertures such that theycan anchor the soot. For example, it is possible to employ elementsformed by two or more different layers, which may also be made ofdifferent materials, wherein the layer facing the filter outlet ismanufactured with a solid material or with a material having arelatively low porosity, in order to form a soot blocking layer, whereasthe layer facing the filter inlet may be comprised of fine metal nets,sintered metallic powders or metallic meshes of a porosity greater thanthat of the soot blocking layer, e.g., with a porosity size similar toor greater than that of the elements employed for the catalyticconverter.

The invention will be further illustrated by the following examples.These non-limiting examples show some embodiments intended to teachthose skilled in the art how to practice the invention and to illustratethe best mode intended to carry out the invention.

EXAMPLE 1 (COMPARATIVE)

This example relates to a gas conversion system not according to theinvention.

A mesh 0.7 mm thick of sintered Fecralloy fibers having a diameter of 35micrometers (μm) is provided. The mesh has a 90% porosity, porositymeaning the ratio of vacuum volume to the geometric volume of the mesh.Three disks having a diameter of 70 mm are cut from this mesh, andtreated at 950° C. in air for 27 hours, thus obtaining the formation ofalumina oxide on the surface of the fibers. The disks so treated areimmersed for 15 minutes into a suspension of AlOOH particles (Disperal®suspension, sold by the SASOL Company of Milan, Italy), then withdrawnfrom the suspension and treated at 700° C. in air for 6 hours. A coatingof porous alumina is obtained, having a weight of 12% of the weight ofthe starting mesh (as measured by the weight difference before and afterthe latter treatment). The disks are then subjected to a catalyzationprocess, immersing them into a platinum nitrate solution at aconcentration of 80 mg/l and leaving them in the solution for 6 hours.At the end of this step, the porous alumina has almost completelyabsorbed the platinum compound from the solution, the disks arewithdrawn from the solution and are subjected to a treatment ofreduction with hydrogen at 350° C. for 3 hours. The amount of metallicplatinum so deposited on the surface of the samples, measured bydifference through ICP analysis of the residual platinum nitratesolution, is equal to 8% of the weight of alumina.

The three disks so produced are arranged in a cylindrical housing havinga circular cross-section, made of AISI 316L steel and having an innerdiameter of 70 mm, the disks being spaced 4 mm apart by spacer rings ofthe type shown in FIG. 6 (elements 64). The thickness of the rings is 3mm, so that the useful diameter of the disks is 64 mm. This system doesnot embody the invention, as each of the disks completely takes up thecross-section of the housing.

The system so formed is employed for an efficiency test of gasconversion. The experimental set-up includes the gas treatment system, aline supplying the gas, a thermocouple at the inlet of the gas treatmentsystem for measuring its inlet temperature, a MKS Baratron 223Bdifferential pressure meter connected to the gas line at the two ends ofthe system under measure, and a Uras 14 gas analyzer by ABB S.p.A. ofSesto San Giovanni (Milan, Italy) downstream of the gas treatmentsystem, for the continuous measurement of the concentration of carbonmonoxide (CO) exiting the system.

Through the system there are passed 40 Nm³/h of a gaseous mixturecomprised of dehumidified air added with 420 parts per million in volume(ppmv) of CO, such a concentration being considered in the field asrepresentative of the gaseous emissions of an internal combustionengine. The system is heated at 350° C. in 10 minutes and left at thistemperature for an additional 10 minutes, while measuring throughout thetest the head loss at the ends of the system and the concentration of COat the outlet thereof. From these measurements the “light off”temperature is further obtained, i.e., the temperature at which theconversion of CO into CO₂ reaches 50%, which is a standard parameter inthe evaluation of systems for the treatment of gases emitted by theengines. The percentage of CO conversion and the head loss (ΔP) inhectoPascal (hPa) at the ends of the system at the end of the test, aswell as the light off temperature, are set forth in Table 1.

EXAMPLE 2

The test of Example 1 is repeated on a system of the invention. In thiscase, the system includes four disks of catalyzed mesh, absolutelyidentical as to materials and preparation to those of Example 1, withthe only difference being that in this case the disks are perforated. Inparticular, each disk is provided with one or more holes, such that thetotal area of the holes on each disk is 25% of the total area of thedisk, the first and the third disks (starting from the gas inlet side)exhibiting a single central hole (disks of type 72 in FIG. 7), whereasthe second and the fourth disks each have sixteen circular holes alongthe edge area (apertures of type 75), the centers of which areequidistant and arranged on a circle 27.5 mm in radius and concentricwith the disk. With this geometric arrangement, the angle α between thediscontinuities in two successive disks, as previously defined, is about62°. This system comprises four disks instead of the three disks of thesystem of Example 1, in order to have the same mesh surface and the sameamount and mean distribution of catalyst, so that the comparison of thetwo conversion tests is as homogeneous as possible (considering that thefour 25% perforated disks have a surface equal to three solid disks).

The gas conversion test is carried out on this system under the sameconditions described in Example 1. The results are set forth in Table 1.

TABLE 1 Test CO conversion (%) ΔP (hPa) Light off T (° C.) 1 >95 24.6171 2 >95 15.8 161

EXAMPLE 3 (COMPARATIVE)

This example relates to a particulate filtration system not according tothe invention.

Two disks having a 70 mm diameter are obtained from a 1.3 mm thick meshof Bekipor® ST XL562 material (produced by the company Bekaert S.A. ofZwevegem, Belgium), made up of three layers of metallic fibers ofdifferent diameter, in particular 17 μm on one side of the mesh, 22 μmin the central layer and 35 μm on the opposite side of the mesh, with atotal porosity of 85%. The two disks are placed in a housing identicalto the one used in Example 1, the side of the mesh formed of the largerfibers facing the inlet of the system. The disks are spaced 4 mm apartby a ring 3 mm thick, leaving a useful mesh surface with a diameter of64 mm.

The system is introduced into the experimental set-up schematized inFIG. 8 and comprised of an air compressor 80, connected through a gasline L₁ to the system 81 whose properties must be measured, and to anabsolute filter 82 downstream of the system (Avasan filter, by ParkerHannifin Company, Corsico, Italy), the filter being able to retainparticles with a diameter greater than 10 μm. At the two ends of system81 there are connected a MKS Baratron 223B differential pressure-meter83 with a reading scale between 0 and 100 hPa, and two absolutepressure-meters 84 with a reading scale between 0 and 3×10⁵ Pa (by thecompany Ashcroft GmbH of Baesweiler, Germany), for measuring head lossvalues greater than 100 hPa.

Along line L₁, upstream of system 81, a by-pass line L₂ is provided,connected to line L₁ through two three-way valves V₁ and V₂. On line L₂a loading chamber 85 for the particulate is present, made of transparentplastic, provided with an airtight lock 86. The particulate used for thetest is Vulcan XC 72 R by the company Cabot Italiana S.p.A. of Ravenna,Italy, with a particle size of about 10 μm, which is one of the standardsynthetic particulates employed for filtration capacity tests in thefield of automotive filters.

At the beginning of the test 40 Nm³/h of dry air are flowed along lineL₁. At the same time 0.2 g of particulate are loaded into chamber 85.Then, by acting on valves V₁ and V₂, the flow is deviated along line L₂,thus causing the air stream to pass through chamber 85. When the totaldisappearance of the particulate from the chamber is observed, the flowis deviated along line L₁ again. The procedure is repeated five times,until obtaining a total loading of 1 g of particulate on filters 81 and82. At the end of the test, the filtration efficiency of the system ismeasured, as a percentage of the particulate retained by system 81 (forconvenience, this percentage is calculated by the weight difference ofabsolute filter 82 before and after the test), and the head loss at theends of system 81. The results of the test are set forth in Table 2.

EXAMPLE 4

The test of Example 3 is repeated but using in manufacturing the system81 four disks of Bekipor® mesh in which circular segments have beenremoved in order to obtain filtering elements of the type shown in FIG.6 (elements 63), such that the area of each of these elements is equalto 65% of the area of the full disk from which they have been obtained.The four disks are placed into the housing, spaced 4 mm apart, in thearrangement shown in FIG. 6, i.e., with the straight edges of theelements parallel to each other and alternated with respect to the axisof the housing. By this geometric construction, the angle α between twosuccessive discontinuities, as previously defined, is about 75°. In thiscase four disks are used, in order to have a particulate retentionefficiency comparable to that of Example 3. The results of the test areset forth in Table 2.

TABLE 2 Test Filtration efficiency (%) ΔP (hPa) 3 37.8 155 4 44.0 68

The comparison of the CO conversion tests (Examples 1 and 2) shows thata system of the invention having the same conversion efficiency of asystem comprised of full disks, has a head loss about 35% lower and alight off temperature 10° C. lower. The comparison between filtrationefficiency tests (Examples 3 and 4) shows that with similar filtrationefficiencies a filter of the present invention exhibits a head loss atits ends which is about 56% lower than that of a filter made of elementsthat completely take up the cross-section of the housing.

It was not possible to carry out a comparison test with four full disks,because a first attempt in that way showed that the filter, gettingclogged by particulate, had head loss values unacceptable for theintegrity of the system (approximately over 500 hPa). Since the headloss linearly increases with the number of filtering elements, and thefilters of the invention have a head loss much lower than the full diskfilters, they allow a remarkable improvement of the filtrationefficiency, yet remaining at acceptable head loss values for theintended applications of these systems, e.g., not greater than 200-300hPa prior to a regeneration in the case of an automotive filter.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1-17. (canceled)
 18. A fumes treatment system comprising a housinghaving a fumes inlet and an outlet for treated fumes, and a series ofessentially planar flow interception elements inside the housing,wherein the elements are formed and arranged such that: a) at least twosuccessive interception elements of the series each has at least onediscontinuity, such that a geometrical area of the discontinuitycomprises between 10 and 40% of an area that the element would have ifit completely occupied an internal cross-section of the housing; b) forany pair of successive interception elements, one being upstream and onebeing downstream of the other and each having a discontinuity, aprojection (P) of the discontinuity of the downstream element on theupstream element, wherein the projection is made perpendicular to theupstream element, does not superimpose with the discontinuity on theupstream element; and c) a minimum distance between two discontinuitieson two successive elements lies along a segment forming an angle α of atleast 5° with respect to a straight line perpendicular to the upstreamelement.
 19. The system according to claim 18, wherein the angle α isgreater than 15°.
 20. The system according to claim 18, wherein theangle α is greater than 30°.
 21. The system according to claim 18,wherein the housing has a circular or elliptical shape in across-section perpendicular to an axis of the housing.
 22. The systemaccording to claim 18, wherein all of the flow interception elementshave discontinuities.
 23. The system according to claim 18, wherein theflow interception elements are all essentially perpendicular to an axisof the housing and essentially parallel and equidistant to each other,wherein each of the elements has discontinuities, and wherein the angleα is lower than 90°.
 24. The system according to claim 23, wherein theangle α is lower than 85°.
 25. The system according to claim 23, whereinthe flow interception elements are retained in a desired position byhollow cylindrical metal parts having a height equal to a desireddistance between two successive elements.
 26. The system according toclaim 23, wherein the housing has a circular cross-section, the flowinterception elements exhibit discontinuities at an internal wall of thehousing in a form of segments missing from a circle which wouldcompletely take up a cross-section of the housing, and the elements areoriented such that chords defining the segments are all parallel to eachother and alternated in successive elements with respect to the axis ofthe housing.
 27. The system according to claim 23, wherein two types ofinterception elements are present in the housing, a first type havingthe discontinuity in a form of a central aperture and a second typewherein the discontinuity has a form of at least one aperture in aperipheral area of the element, and wherein elements of the two typesare arranged in an alternating manner along the axis of the housing. 28.The system according to claim 23, wherein the angle α between twosuccessive flow interception elements is constant along the axis of thehousing.
 29. The system according to claim 23, wherein the angle αbetween two successive flow interception elements increases along theaxis of the housing going from the inlet to the outlet of the housing.30. The system according to claim 18, wherein the flow interceptionelements comprise a porous metal material having a form selected from afiber mesh, a porous body of sintered powders, a foam, and combinationsof these forms.
 31. The system according to claim 30, wherein theelements comprise Fecralloy alloy or AISI 310S steel.
 32. The systemaccording to claim 30, wherein the elements are mats comprisingmultilayers of metal fibers, wherein the layers have differentporosities and are formed of fibers of different size.
 33. The systemaccording to claim 18, wherein the flow interception elements comprise aceramic material selected among silicon carbide, cordierite and aluminumtitanate.
 34. The system according to claim 18, for use as a particulatefilter, wherein the flow interception elements comprise two layers ofdifferent materials, one layer of solid material and one layer of amaterial selected from fine metal nets, sintered metal powders and metalfiber meshes.